This invention relates to a ball screw used in various feeding devices, and more particularly to a ball screw having spacers, and to a method for inserting balls and spacers into the ball screw.
A typical known ball screw device comprises a screw shaft having a spiral ball rolling groove on its outer peripheral surface; a ball nut having a spiral ball rolling groove on its inner peripheral surface, the ball rolling groove of the ball nut facing and being opposed to the ball rolling groove of the screw shaft, whereby the ball rolling grooves together form a spiral ball rolling channel having first and second ends; a ball circulating passage connecting said ends of the ball rolling channel; and a plurality of balls in the ball rolling channel and in the ball circulating passage. For example, the ball screw device of FIG. 10, known end-cap system ball screw device, comprises a screw shaft 40 having an outer peripheral surface on which a spiral ball rolling groove 42 is formed, a ball nut 50 having an inner peripheral surface on which a spiral ball rolling groove 52 is formed. The groove 52 is opposed to, and faces, groove 42, forming the spiral ball rolling channel. The ball nut 50 has a thick wall in which an axially extending ball return passage 54 is formed. End caps 60 are mounted on the axial end surfaces of the ball nut, and ball guide grooves 62 in the end caps connect the spiral channel to the ends of the axial ball return passage, the grooves 62 and axial passage 54 thereby forming a ball circulating passage 58. Balls 70 are situated in the channel formed by the opposed grooves, and in the ball circulating passage.
Besides the above-mentioned end-cap ball recirculating scheme, other recirculating schemes have been used, including ball return tubes, barrels and guide plate systems. These are described in Izawa, Minoru, Ball Screw Application Technique First Ed., Kogyo Chosakai Co. Ltd., May 20, 1993, pages 19-21.
Most such ball screw devices adopt the xe2x80x9call ballxe2x80x9d specification, meaning that the balls are disposed close to one another. The balls which support the load are large in number and hence the ball screw device has a large load-bearing capability and good rigidity. However, because of nonuniformities in the shape of the ball rolling grooves and the like, there are slight differences in the revolving speeds of the balls. In the load region, when the revolving speed of a rear ball is larger than that of a front ball as seen in the advancing direction of the balls, the rear ball bumps into the front ball, these balls jostle each other, and a compressive force is liable to act at the point at which the jostling balls contact each other.
When the compressive force acts at the contact point between two balls, sliding contact acts in a direction to prevent rolling of the balls. Consequently a large resistance is generated, which prevents rotation of the balls, giving rise to fluctuation or a remarkable increase in the dynamic torque of the ball screw device. Furthermore, a ball clogging phenomenon may occur, and sliding contact of the balls also causes problems with the generation of noise (usually expressed by xe2x80x9csound pressure levelxe2x80x9d) and rapid wear of the balls.
The increase in dynamic torque during low speed movement or oscillatory movement is considered to be due primarily to a phenomenon, occurring as a result of sliding contact due to jostling between load-supporting balls, in which the balls cut into the ball rolling groove surfaces.
To alleviate the above-mentioned problems in devices made in accordance with the xe2x80x9call ballxe2x80x9d specification, the number of inserted balls is typically made two to four less than the full number of balls that could be inserted. The reduction in the number of balls provides a clearance between balls in the load region, in order to reduce jostling between balls.
In order to reduce fluctuations in dynamic torque, and particularly to reduce the remarkable fluctuations and increase in dynamic torque generated during low speed movement or during repeated reciprocating movement, and to reduce tilting movements occurring during minute feeding, spacer balls are used. The spacer balls have a diameter slightly less (by several tens of xcexcm) than the diameter of the load bearing balls. When a spacer ball comes into rolling contact with a load-bearing ball, they roll in opposite directions. Accordingly, the sliding contact generated in the case of a ball screw device in accordance with the xe2x80x9call ballxe2x80x9d specification is avoided, and the resulting resistance to rolling becomes extremely small, and minimum fluctuation of dynamic torque can be achieved.
Another proposed solution to the aforementioned problems it""s the use of a resilient, strip-like retainer which respectively holds balls rotatably in a large number of ball pockets, as depicted in Japanese laid-open utility model publication 27408/1993. The retainer strip is capable of circulating movement.
Similarly, another ball screw device has been proposed in which a resilient ball chain and connector belt are used, as depicted in Japanese laid-open patent publication 169746/1998. The resilient ball chain consists of a large number of balls arranged in line at a given interval, and the connector belt rotatably holds these balls and connects neighboring balls with one another. The ball chain and connector belt are capable of circulating movement.
In both of the last two approaches mentioned above, contact between the balls can be prevented, and consequently fluctuations in dynamic torque can be reduced and ball clogging can be prevented. Moreover, noises due to sliding contact between the balls, and rapid wear of the balls can be prevented.
Still another proposal has been to utilize spacers with spherical concave surfaces on both axial end surfaces thereof. The spacers, which are uniform in thickness are disposed between adjacent balls and portions of the spherical concave surfaces of the balls fit slidably into the concave faces of the spacers, as shown in Japanese laid-open utility model publication 178659/1988. The spacers are all of the same thickness, the term xe2x80x9cthicknessxe2x80x9d referring to the dimension equal to the distance between vertices of the adjacent balls when the adjacent balls are in close contact with the concave surfaces of the spacer between them. In other words, thickness of a spacer is the distance of closest approach of the surfaces of the balls when separated by a spacer.
The ball screw device utilizing spacers avoids point contact between balls, utilizing instead face contact between the balls and the concave surfaces of the spacers. Thus, the high pressure inherent in point contact in the ball screw devices according to the xe2x80x9call ballxe2x80x9d or xe2x80x9cspacer ballxe2x80x9d specifications is avoided. Furthermore, the spacers allow the distance between load-supporting balls to be small compared with devices made according to the xe2x80x9cspacer ballxe2x80x9d specification, and consequently ball screw devices utilizing spacers generally have a greater load-bearing capability and greater rigidity than those utilizing spacer balls.
The above-described ball screw devices have various drawbacks, which will be described below.
The ball screw device in which the number of balls is from two to four fewer than the number which would fully load the ball rolling channel and ball circulation passage is subject to ball clogging. The ball clogging phenomenon results from the remarkable increase in dynamic torque which is caused by the jostling of the load-supporting balls when the device is subjected to low speed movement or oscillatory movement.
In the ball screw device in accordance with the spacer ball specification, the load capacity and rigidity of the device are reduced as a result of the decrease in the number of load-supporting balls. For example, when the ratio of load supporting balls to spacer balls is 1:1, the basic dynamic rated load indicative of the load capacity is reduced to approximately 60%. Further, since the balls which support the load and the spacer balls are in contact with one another while rolling in opposite directions, the device has a tendency to generate noise and cause wear of the balls due to the contact between the load-bearing balls and the spacer balls.
In the case of the ball screw devices using a strip-like retainer or a ball chain, guiding grooves must be formed in the wall surface of the ball circulating passage, necessitating an additional machining operation and increasing the manufacturing cost.
In addition, the strip-like retainer of ball chain is manufactured after determining its total length preliminarily. However, it is difficult to make the total length of the ball rolling channel and the ball circulating passage equal to the total length of the strip-like retainer or ball chain. Accordingly, when the latter is shorter than the former, a clearance is formed between the leading and trailing ends of the strip-like retainer or ball chain. On the other hand, when the latter is longer than the former, it is impossible to insert the strip-like retainer or ball chain into the ball rolling channel and circulating passage.
When a strip-like retainer or ball chain is wound around the ball screw in the spiral ball rolling channel, it is twisted or bent on an axis extending through the centers of the balls. As a result, when the balls revolve and roll while supporting a load, there is a high probability that breakage will occur due to twisting or bending of the strip-like retainer or ball chain. Also, since the ball circulating passage is bent sharply, there is a possibility that the strip-like retainer or ball chain will break in the ball circulating passage.
In the case of a ball screw device incorporating spacers of uniform thickness between neighboring balls, various other problems arise.
Spacers are manufactured for each model of ball screw device after a preliminary determination of the spacer thickness needed to avoid a clearance between the balls. However, even in the case of a single model of ball screw device, machining errors such as errors in the effective diameters of the ball rolling grooves, and differences in ball diameters, result in irregularities in the total length of the ball rolling channels and ball circulating passages. Accordingly, when the balls and spacers are inserted alternately against one another into the ball rolling channel and the ball circulating passage, a clearance is formed between the ball which is inserted first and the spacer which is inserted last. Depending on the dimension of this clearance, when the screw shaft or ball nut is rotated, the dynamic torque may fluctuate or remarkably increase. In addition, the circulation of the balls may be interrupted due to tilting of the spacers. The use of spacers having a greater thickness will decrease the number of load-bearing balls that can be accommodated in the ball rolling channel, with a resultant decrease in the load-carrying capacity and rigidity of the ball screw device.
As shown schematically in FIG. 11, balls 70a and 70b, which are load supporting balls, move and roll within a load region between ball rolling grooves 42 and 52. Here, as in the case of a ball screw device according to the xe2x80x9call ballxe2x80x9d specification, when the revolving speed of the rear ball 70b is greater than the revolving speed of the front ball 70a, the spherical surfaces of the balls come into close contact with concave surfaces 82 of the spacer 80. The balls jostle each other by way of the spacer 80, and the concave surfaces 82 of the spacer are liable to be sandwiched by wide contact areas. Then, when sandwiching forces P act on the concave surfaces 82 of the spacer, a sliding contact is formed over a wide area in a direction to prevent rolling of the balls. Large resistances Q, which obstruct the rolling of the balls 70a and 70b are generated and the dynamic torque of the device is increased.
When the magnitude of the resistances Q becomes equal to or greater than the force needed to roll the balls 70a and 70b, the balls cannot roll and come into sliding contact with grooves 42 and 52. The consequence is that dynamic torque is dramatically increased and ball clogging occurs. The phenomenon of ball clogging is especially likely to occur during low speed movement or when the ball screw device is subjected to tilting.
T indicates the thickness of the spacer 80, which is equal to the distance of closest approach between the balls 70a and 70b when the spherical surfaces of the balls are in close contact with the concave surfaces of the spacer.
The invention addresses the above-discussed problems. In particular, it is an object of the invention to provide a ball screw device having spacers, which makes the balls and spacers circulate smoothly without generating significant fluctuations in, or increasing, the dynamic torque of the device. Another object of the invention is to provide a method for inserting the balls and spacers of such a ball screw device.
A further object of the invention is to provide a ball screw device having spacers, which has a load capacity and rigidity approaching those of a conventional xe2x80x9call ballxe2x80x9d device having a number of balls two to four fewer than a full load.
A still further object is to prevent the fluctuation and increase in dynamic torque which tend to occur when both concave surfaces of a spacer are in close contact with, and sandwiched by, neighboring balls, and differences in revolving speeds of the balls occur, caused for example by non-uniformity in the shape of the ball rolling grooves.
To achieve the aforementioned objects, a preferred ball screw device in accordance with the invention comprises a screw shaft having an outer peripheral surface and a spiral ball rolling groove on said outer peripheral surface and a ball nut having an inner peripheral surface and a spiral ball rolling groove on said inner peripheral surface, the ball rolling groove of the ball nut facing and being opposed to the ball rolling groove of the screw shaft, whereby the ball rolling grooves together form a spiral ball rolling channel having first and second ends. A ball circulating passage connects the ends of the ball rolling channel. The device has a plurality of balls in the ball rolling channel and in the ball circulating passage. Spacers are disposed between adjacent balls, each of the spacers having spherical concave end surfaces facing in opposite directions on an axis about which the concave surfaces are symmetrical. The concave end surfaces of each spacer, receive and slidably fit portions of the spherical outer surfaces of two adjacent balls of said plurality of balls, respectively. The improvement comprises discontinuities in the concave surfaces of the spacers. The discontinuities may be constituted by a plurality of protrusions formed on the concave surfaces of said spacers, the protrusions on the concave surface of each spacer being distributed and having ends complementary to the spherical outer surface of the ball received therein. Alternatively, the discontinuities may be constituted by dimples formed in the concave surfaces of said spacers, the dimples in the concave surface of each spacer being distributed.
Preferably the spacers are of two different kinds. The spacers of a first group have a first thickness and the spacers of a second group have a second thickness. The thicknesses of the spacers are such that the distance of closest approach of some adjacent balls to each other is greater than the distance of closest approach of other adjacent balls to each other, and both of said distances are greater than zero.
The thickness T2 of the spacers of the first group is preferably related to the thickness T1 of the spacers of the second group by the formula T2=T1+D/k, where D is the ball diameter, and k is a constant.
The thickness T1 of the spacers of the second group should be such that the closest approach of adjacent balls on opposite sides thereof to each other is in the range of 0.2 to 0.5 mm.
It is preferable that a preload is applied between the balls and both ball rolling grooves.
It is also preferable that the spacers have a column-like or disc-like shape and the diameter of the spacers is in the range of 60% to 80% of the diameter of the balls.
In a preferred embodiment of the invention, the spacers have concave outer peripheral surfaces surrounding an axis extending along the path of the balls through the ball rolling channel and the ball circulating passage.
The spacers are also preferably made of a self-lubricating material, a plastics material containing a lubricant or a plastics material impregnated with a lubricant.
The balls preferably have innumerable, minute, needle-like recesses formed in, and randomly distributed on, their spherical outer surfaces.
A method for inserting balls and spacers into a ball screw device is another aspect of the invention. The method is applicable to a ball screw device comprising a screw shaft having an outer peripheral surface and a spiral ball rolling groove on said outer peripheral surface; a ball nut having an inner peripheral surface and a spiral ball rolling groove on said inner peripheral surface, the ball rolling groove of the ball nut facing and being opposed to the ball rolling groove of the screw shaft, whereby the ball rolling grooves together form a spiral ball rolling channel having first and second ends; a ball circulating passage connecting said ends of the ball rolling channel; a plurality of balls in the ball rolling channel and in the ball circulating passage, the balls having spherical outer surfaces; and spacers disposed between adjacent balls, each of the spacers having spherical concave end surfaces facing in opposite directions on an axis about which the concave surfaces are symmetrical, said concave end surfaces of each spacer, receiving and slidably fitting portions of the spherical outer surfaces of two adjacent balls of said plurality of balls, respectively.
In accordance with the method, the balls and spacers are inserted alternately into the path consisting of the ball rolling channel and the ball circulating passage. When there are no clearances between the successively inserted balls and spacers, the clearance between the ball which is inserted first and the thinnest portion of the spacer which is inserted last is preferably in the range of ⅓ to xc2xd of the diameter of the balls which are inserted.
The method of the invention is preferably carried out using two groups of spacers, the spacers of a first group having a first thickness and the spacers of a second group having a second thickness. The thicknesses of the spacers should be such that the distance T2 of closest approach of some adjacent balls to each other is greater than the distance T1 of closest approach of other adjacent balls to each other, and both of the distances T2 and T1 are greater than zero.
The thickness T2 of the spacers of the first group is preferably related to the thickness T1 of the spacers of the second group by the formula T2=T1+D/k, where D is the ball diameter, and k is a constant.
In carrying out the insertion method, a number N of balls, a number n1 of spacers of the second group, and a number n2 of spacers of the first group, are inserted into the ball rolling channel and the ball circulation passage, which together have a total length L. The clearance between the ball which is inserted first and the thinnest portion of the spacer which is inserted last has a value C, which is greater than zero. The values D, N, T1, T2, n1 and n2 satisfy the following relationships:
L=Dxc2x7N+T1xc2x7n1+T2xc2x7n2+(⅓ to xc2xd)xc2x7D
and
N=n1+n2,
and n2 is the integer portion of the following numbers:
in the case in which (⅓ to xc2xd)xc2x7D less than C, n2=(Cxe2x88x92(⅓ to xc2xd)xc2x7D)/(T2xe2x88x92T1),
and
in the case in which (⅓ to xc2xd)xc2x7D greater than C, n2=((C+D+T1)xe2x88x92(⅓ to {fraction (1/2)})xc2x7D)/(T2xe2x88x92T1).
Again, in carrying out the method the thickness T1 should be in the range from 0.2 to 0.5 mm, and a preload should be applied between the balls and the ball rolling grooves.
When the clearance between the ball which is inserted first and the thinnest portion of the spacer which is inserted last is in the range of ⅓ to xc2xd of the diameter of the balls which are inserted, the following advantages are obtained.
First, when the screw shaft or ball nut is rotated, if the revolving speeds of the balls are uniform at the time that they move and roll, the spacers are held so that the spherical surfaces of the neighboring balls and the radially outward portions of the concave surfaces of the spacers are brought into contact with each other while clearances are maintained between the spherical surfaces of the balls and the concave surfaces of the spacers so that fluctuations and increases in dynamic torque are prevented and smooth circulation of the balls and spacers is ensured.
Second, when a lubricant (grease or oil) fills the space between the ball rolling grooves, clearances defined between the spherical surfaces of the balls and the concave surfaces of the spacers form oil pockets enhancing the retention of lubricant.
Third, by disposing spacers between the balls which set the clearance between balls to a dimension in the range from xc2xd to ⅓ of the ball diameter the number of balls can be within the range of 2 to 4 balls less than the full number of balls that can be loaded. Consequently, the ball screw device of the invention can achieve a load capacity and rigidity similar to those of an xe2x80x9call ballxe2x80x9d device.
The protrusions and dimples formed on the concave surfaces of the spacers give rise to the following further advantages.
First, even when both concave surfaces of the spacer are brought into close contact with the spherical surfaces of the balls, and the spacers are sandwiched between balls revolving at different speeds, the resistances that obstruct the rolling of the balls is relatively small, and consequently the dynamic torque neither fluctuates nor increases. By applying a preload between the balls and both ball rolling grooves, a frictional force generated by the preload is added to the force needed to roll the balls, and hence the balls can roll more easily.
Secondly, even when the ball screw device is subjected to low speed movement or oscillatory movement, it is possible to prevent the remarkable increase in dynamic torque and to avoid the resulting ball clogging phenomenon.
Third, when the ball rolling grooves are filled with a lubricant, the spaces defined between the protrusions, or, in the case of dimples, the dimples themselves, function as oil pockets for the lubricant. Hence, the formation of oil films on the spherical surfaces of the balls is not interrupted, and wear of the balls and the ball rolling grooves, caused by point contact between the balls and the grooves, can be prevented.
Since the spacers are not connected to one another, and each spacer is disposed between independent neighboring balls, the following advantages are realized.
First, since the balls to not come into contact with each other, the invention avoids wear and the generation of noise which occur in conventional xe2x80x9call ballxe2x80x9d or xe2x80x9cspacer ballxe2x80x9d devices as a result of contact between adjacent balls.
Second, the invention obviates the extra machining steps needed for example to form recessed guide grooves in the ball circulating passage in strip retainer or ball chain devices. Accordingly, the ball screw device can flexibly cope with individual products including conventional products.
Embodiments of a method for inserting balls and spacers in a ball screw device according to the invention, and a ball screw device produced by the method, are explained in the following detailed description in conjunction with the drawings.